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Rigid foam insulation is a type of insulation that comes in pre-made panels. It can be used to insulate the roof, foundation, or outer walls of a building. This type of insulation is a good choice for buildings with limited space for insulation that still need high thermal resistance.
The effectiveness of insulation is notated using resistance value (R value). R value is the insulation's ability to resist the flow of hot and cold air. The higher the R value, the greater the temperature protection. This value varies based on the climate in which the insulation is installed.
The R value of rigid foam insulation ranges from R3 to R6.5. This is nearly twice the R value attributed to other types of insulation of the same thickness. Rigid foam insulation is installed as an unbroken sheet on the outside of the building, so it increases the overall R value of the structure.
Wood is not a good insulator. Many modern homes are wood frame structures, and insulation is often installed between the studs of a wood frame house. This means that, according to some experts, about 25 percent of the average house is insulated only by wood.
After a house has been framed out, but before any finishing or siding is added, rigid foam insulation can be applied to the outside of the house. It can also be used to cover the outside of roofs or foundations. Panels are placed tightly side by side and held to the wall using glue or nails. Seems and holes are then taped to prevent air leaks. This forms a solid coat of insulation over the whole structure.
The R value of the rigid foam insulation is dependent on the material used to make the panels. It can be made of expanded polystyrene, extruded polystyrene, or polyisocyanurate. The most commonly used of these is expanded polystyrene. It is the least expensive but also has the lowest R value. It also has no facing, so it is not water resistant.
The most expensive of the three, polyisocyanurate insulation also has the highest R value. Construction crews start with a liquid foam which they spray on to a backing to create rigid foam insulation panels. If foil facing is used, the insulation can create a watertight barrier for the house.
Extruded polystyrene panels are the middle range of rigid foam insulation. Their cost and R value fall halfway between the other two varieties. These panels can be faced or unfaced. Faced panels form a moisture resistant barrier for the house.